An electronic image capturing device, such as a digital still camera, generally includes an autofocus (AF) unit that automatically focuses on an object. As an AF control method for the AF unit, hill-climbing AF control is widely used (see, for example, Japanese Examined Patent Application Publication No. S39-5265). The hill-climbing AF control is performed by calculating an integral of differences in luminance of adjacent pixels based on a video signal output from an image sensor and by using the integral of the differences in luminance as an AF evaluation value for evaluating a degree of focusing. At an in-focus state, edge portion of an object is highly contrasting, and hence the differences in luminance of the neighboring pixels are large; accordingly, the AF evaluation value is large. In contrast, in an out-of-focus state, the edge portion of the object is blurred, and hence the differences in luminance of the neighboring pixels are small; accordingly, the AF evaluation value is small. During AF operation, the AF unit causes a lens to move while obtaining a sequence of AF evaluation values one after another, and causes the lens to stop at a position serving as an in-focus position where the AF evaluation value is maximized, or, put another way, at a peak position.
In recent years, there is provided a technology for implementing a function effective for a scene that includes a large number of focal distances in a view as in a case of taking a close-up photograph of flowers. This technology allows a user to select an image focused on a desired area after shooting by causing a camera to automatically determine focal distances from the camera to multiple points based on information on objects, perform high-speed continuous shooting while focusing is shifted to each of the distances, and store resultant images of the shooting as electronic files in the Multi-Picture Format.
Disclosed in Japanese Patent Application Laid-open No. H04-158322 is a technology (automatic tracking feature) for obtaining an average value (center of gravity) of luminance distribution in a focus area, detecting movement of an object by comparing the average values between different frames, causing the focus area to shift to a new focus area by tracking the movement, and detecting an in-focus position in the new focus area.
Proposed in Japanese Patent Application Laid-open No. 2010-128395 is an image capturing device that determines an autofocus area based on a line designated by a user on a screen so that a focusing-target object can be designated accurately.
However, according to the conventional AF technology disclosed in Japanese Examined Patent Application Publication No. S39-5265 or the like, AF range-finding scanning is performed prior to high-speed continuous shooting. It is already known that this AF range-finding scanning is time consuming because the AF range-finding scanning is performed by obtaining AF evaluation values while a focusing lens is moved over an entire area where the focusing lens is movable and selecting peak positions one after another.
The technology disclosed in Japanese Patent Application Laid-open No. H04-158322 or No. 2010-128395 that allows designating an object to be focused is also time consuming in processing time involved in auto focusing, which can result in losing a good opportunity for shooting a photograph.
The present invention has been conceived in view of the above circumstances and provides an image capturing device and an image capturing method with which a user can continuously take desired photographs at desired instants associated with fast range-finding processes.